The need for such applications as ever increasingly smaller critical dimensions for semiconductor integrated circuit manufacturing the need has arisen to move from the generation of Deep Ultraviolet (“DUV”) light to Extreme Ultraviolet (“EUV”) light, also referred to as soft-x-ray light. Various proposals exist for apparatus and methods for the generation of such light at effective energy levels to enable, e.g., adequate throughput in an EUV lithography tool (e.g., a stepper scanner or scanner) over an acceptable lifetime between, e.g., replacements of major components.
Proposals exist for generating, e.g., light centered at a wavelength of 13.5 nm using, e.g., Lithium which is introduced into and/or irradiated to form a plasma which excites the lithium atoms to states from which decay results in large part in EUV light photons having an energy distribution centered about 13.5 nm. The plasma may be formed by an electrical discharge using a dense plasma focus electrode in the vicinity of a source of lithium in solid or liquid form, e.g., as discussed in U.S. Pat. Nos. 6,586,757, entitled PLASMA FOCUS LIGHT SOURCE WITH ACTIVE BUFFER GAS CONTROL, issued to Melynchuk et al. on Jul. 1, 2003, and the above referenced patent application Ser. No. 10/409,254 filed Apr. 8, 2003, and U.S. Pat. No. 6,566,668, entitled PLASMA FOCUS LIGHT SOURCE WITH TANDEM ELLIPSOIDAL MIRROR UNITS, issued to Rauch et al. on May 20, 2003, and U.S. Pat. No. 6,566,667, entitled PLASMA FOCUS LIGHT SOURCE WITH IMPROVED PULSE POWER SYSTEM, issued to Partlo et al on May 20, 2003, which are assigned to the assignee of the present application and applications and patents and other references referenced therein, the disclosures of all of which are hereby incorporated by reference, and also other representative patents or published applications, e.g., United States Published application No. 2002-0009176A1, entitled X-RAY EXPOSURE APPARATUS, published on Jan. 24, 2002, with inventors Amemlya et al. the disclosures of which are hereby incorporated by reference. In addition, as noted in, e.g., patents and published applications U.S. Pat. No. 6,285,743, entitled METHOD AND APPARATUS FOR SOFT X-RAY GENERATION, issued to Kondo et al. on Sep. 4, 2001, U.S. Pat. No. 6,493,423, entitled METHOD OF GENERATING EXTREMELY SHORT-WAVE RADIATION . . . , issued to Bisschops on Dec. 10, 2002, United States Published application 2002-0141536A1 entitled EUV, XUV AND X-RAY WAVELENGTH SOURCES CREATED FROM LASER PLASMA . . . , Published on Oct. 3, 2002, with inventor Richardson, U.S. Pat. No. 6,377,651, entitled LASER PLASMA SOURCE FOR EXTREME ULTRAVIOLET LITHOGRAPHY USING WATER DROPLET TARGET, issued to Richardson et al. on Apr. 23, 2002, U.S. Pat. No. 6,307,913, entitled SHAPED SOURCE OF X-RAY, EXTREME ULTRAVIOLET AND ULTRAVIOLET RADIATION, issued to Foster et al. on Oct. 23, 2001, the disclosures of which are hereby incorporated by reference, the plasma may be induced by irradiating a target, e.g., a droplet of liquid metal, e.g., lithium or a droplet of other material containing a target of, a metal, e.g., lithium within the droplet, in liquid or solid form, with, e.g., a laser focused on the target.
Since the amount of energy in the EUV light desired to be produced within the desired bandwidth, from the creation of such a plasma and resultant generation from the plasma of EUV light, is relatively enormous, e.g., 100 Watts/cm2, its is necessary to ensure that the efficiency of the collection of the EUV light be made as high as possible. It is also required that this efficiency not significantly deteriorate, i.e., be able to sustain such high efficiency, over relatively extended periods of operation, e.g., effectively a year of operation at very high pulse repetition rates (4 KHz and above) for an effective 100% duty cycle. Many challenges exist to being able to meet these goals aspects of which are dealt with in explaining aspects of the disclosed subject matter regarding a collector for an EUV light source.
Some issues that are required to be addressed in a workable design include, e.g., Li diffusion into the layers of a multi layer normal angle of incidence reflecting mirror, e.g., through an outer coating of ruthenium (“Ru”), with the multilayered mirror made, e.g., of alternating layers of Molybdenum (“Mo” or “Moly”) and silicon (“Si”) and the impact on, e.g., the primary and/or secondary collector lifetime; chemical reactions between, e.g., Li and Si and the impact on, e.g., the primary and/or secondary collector lifetimes; scatter of out of band radiation, e.g., from the laser producing the irradiation for ignition to form the plasma, e.g., 248 nm radiation from an KrF excimer laser required to be kept low to avoid any impact on resist exposure given that Deep UV resist types may be carried over into the EUV range of lithography and such out of band light scattered from the target can result in exposing the resist very efficiently; achieving a 100 W delivery of output light energy to the intermediate focus; having a lifetime of a primary and secondary collector of at least 5 G pulses; achieving the required conversion efficiency with a given source, e.g., a given target, e.g., a target droplet or target within a droplet, or other targets, the preservation of lifetime of the required multi layer mirrors at operational elevated temperatures and out of band radiation at center wavelengths near, e.g., 13.5 nm.
It is well known that that normal incidence of reflection (“NIR”) mirrors can be constructed for wavelengths of interest in EUV, e.g., between about 5 and 20 nm, e.g., around 11.3 nm or 13.0-13.5 nm utilizing multi-layer reflection. The properties of such mirrors depend upon the composition, number, order, crystallinity, surface roughness, interdiffusion, period and thickness ratio, amount of annealing and the like for some or all of the layers involved and also, e.g., such things as whether or not diffusion barriers are used and what the material and thickness of the barrier layer is and its impact on the composition of the layers separated by the barrier layer, as discussed, e.g., in Braun, et al., “Multi-component EUV multi-layer mirrors, Proc. SPIE 5037 (2003) (Braun”); Feigl, et al., “Heat resistance of EUV multi-layer mirrors for long-time applications,” Microelectronic Engineering 57-58, p. 3-8 (2001) (“Feigl”), U.S. Pat. No. 6,396,900, entitled MULTILAYER FILMS WITH SHARP, STABLE INTERFACES FOR USE IN EUV AND SOFT X-RAY APPLICATION, issued to Barbee, Jr. et al. on May 28, 2002, based upon an application Ser. No. 10/847,744, filed on May 1, 2002 (“Barbee”) and U.S. Pat. No. 5,319,695, entitled MULTILAYER FILM REFLECTOR FOR SOFT X-RAYS, issued to Itoh et al. on Jun. 7, 1994, based on an application Ser. No. 45,763, filed on Apr. 14, 1993, claiming priority to a Japanese application filed on Apr. 21, 1992 (“Itoh”).
Itoh discusses materials of different X-ray refractive indexes, for example, silicon (Si) and molybdenum (Mo), alternately deposited on a substrate to form a multilayer film composed of silicon and molybdenum layers and a hydrogenated interface layer formed between each pair of adjacent layers. Barbee discusses a thin layer of a third compound, e.g., boron carbide (B4C), placed on both interfaces (Mo-on-Si and Si-on-Mo interface). This third layer comprises boron carbide and other carbon and boron based compounds characterized as having a low absorption in EUV wavelengths and soft X-ray wavelengths. Thus, a multi-layer film comprising alternating layers of Mo and Si includes a thin interlayer of boron carbide (e.g., B4C) and/or boron based compounds between each layer. The interlayer changes the surface (interface) chemistry, which can result in an increase of the reflectance and increased thermal stability, e.g., for Mo/Si where inter-diffusion may be prevented or reduced, resulting in these desired effects. Barbee also discusses varying the thickness of the third layer from the Mo-on-Si interface to the Si-on-Mo interface. Barbee also discusses the fact that typically the sharpness of the Mo-on-Si interface would be about 2.5 times worse than that of the Si-on-Mo interface; however, due to the deposition of the interlayer of B4C in the Mo-on-Si interface, such interface sharpness is comparable to that of the Si-on-Mo interface. Braun discusses the use of carbon barrier layers to reduce inter-diffusion at the Mo—Si boundaries to improve the thermal stability and lower internal stress and at the same time increasing reflectivity. Braun notes that normally the Mo—Si boundary forms MoSi2 at the interface in varying thicknesses at the Mo-on-Si boundary and the Si-on-Mo boundary, and also that the morphology of the Mo and/or Si layers can be influenced by barrier layers of, e.g., carbon content. In addition Braun notes the impact of barrier layer formation on interface roughness of the Mo—Si interface without a barrier layer. Braun reports a reflectance at λ=13.3 nm of 70.1% using Mo/SiC multi-layers. The reduction in internal stress using B4C even with annealing as compared to Mo/Si/C multi-layers, which impacts the ability to uses such multi-layer mirrors for curved mirrors is also discussed. Braun also discusses the tradeoff between interlayer contrast, impacting reflectivity, and absorption in the multi-layer configurations, such that, e.g., NbSi layers with lower absorption in the Nb but also lower contrast, and Ru/Si with higher contrast but also higher absorption in the Ru layer, both performing less effectively than a Mo/Si multi-layer stack. Braun also discusses the theoretical utility of using three layers of, e.g., Mo/Si/Ag or Mo/Si/Ru, which have theoretically higher reflectivity, but that the Ag embodiment fails to achieve the theoretical reflectivity due to voids in the Ag layer at desired thicknesses and a calculated best reflectivity of a Mo/Si/C/Ru multi-layer stack at λ=13.5 nm, with a thickness constrained in the Mo layer to prevent crystallization in the Mo layer. However, Braun also finds that the Mo/Si/C/Ru multi-layer stacks do not live up to theoretical calculated reflectivity expectation, probably due to an initial Mo layer deposition surface roughness that propagates upward through the stack. Feigl discusses the impact of elevated temperatures up to 500° C. on the structural stability of, e.g., Mo/Si and Mo/Mo2C/Si/Mo2C multilayer stacks, including the use of ultra thin Mo2C barrier layers. Feigl notes that the barrier layer prevents the formation of inter-diffusion layers of MoSix due to annealing of the Mo and Si at temperatures above, e.g., 200° C. and that Mo/Mo2C/Si/Mo2C and Mo2C/Si systems remain stable up to 600° C. The former system having ultra thin Mo2C barrier layers (MoSi2 is also suggested but not tested) layers and the latter is formed by substituting Mo2C for Mo in a multilayer system. The reflectivity of the Mo2C/Si system remained above 0.8 through 600° C. according to Feigl, whereas the Mo/Mo2C/Si/Mo2C system tailed off to slightly less than 0.7 at that temperature, and even decreased to about 0.7 at 400° C.
Applicants in the present application propose certain other materials for barrier layers and other potential improvements to the multi-layer stack for EUV applications.